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Genetic Counseling and Genetic Testing Before Pregnancy

(Prenatal Genetic Counseling; Prenatal Genetic Testing)

By

Jeffrey S. Dungan

, MD, Northwestern University, Feinberg School of Medicine

Reviewed/Revised Feb 2024
VIEW PROFESSIONAL VERSION
Topic Resources
  • Hereditary genetic disorders are passed down from generation to generation.

  • Spontaneous genetic disorders are not passed down from one generation to another, but they occur when genetic material in the father’s sperm or mother's egg cells or in the cells of the developing embryo is damaged by chance or by medications, chemicals, or other damaging substances (such as radiation).

  • Before pregnancy, prospective parents should speak with their health care professional about their risk of having a baby with a genetic disorder.

  • Risk factors include older age in the woman or man, a family history of genetic abnormalities, a chromosomal abnormality in one of the prospective parents, a previous miscarriage or stillbirth, or a previous baby with a birth defect.

  • Testing for genetic disorders is offered to all women but is particularly important if a couple’s risk is higher than average.

Prospective parents who are thinking of having a baby should speak with a health care professional about the risks of genetic abnormalities (prenatal genetic counseling). Prospective parents can review their family history and other medical history with their doctor or a genetic counselor to determine whether their risk of having a baby with a hereditary genetic abnormality is higher than average. If so, tests to assess those risks more precisely (genetic carrier screening Genetic Counseling and Genetic Testing Before Pregnancy Genetic disorders are caused by abnormalities in one or more genes or chromosomes. Some genetic disorders are hereditary and others are spontaneous. Hereditary genetic disorders are passed down... read more ) can be done. If genetic screening tests show a prospective parent has a genetic abnormality, the couple and doctor can discuss the options for avoiding having the abnormality pass down to a child.

Types of and Risk Factors for Genetic Disorders

All cells in the human body contain deoxyribonucleic acid (DNA). Genes are segments of DNA that contain the code for a specific protein that functions in one or more types of cells in the body or the code for another type of molecule called ribonucleic acid (RNA). Chromosomes are structures within cells that contain a person's genes.

Genetic disorders can be caused by an abnormality in a single gene, multiple genes, or part or all of a chromosome. The following percentage of all babies have an abnormality:

  • 0.5% have a chromosomal disorder.

  • 1% have a single-gene disorder.

  • 1% have a disorder caused by defects in several genes.

All pregnancies involve some risk of genetic abnormalities. Certain conditions in the parents increase risk, such as advanced age.

Chromosomal abnormalities

Chromosomal abnormalities Overview of Chromosome and Gene Disorders Chromosomes are structures within cells that contain a person's genes. Genes are segments of deoxyribonucleic acid ( DNA) and contain the code for a specific protein that functions in one or... read more , many of which involve an abnormal number of chromosomes or an abnormality in a chromosome's structure, occur in about 1 of 200 live births in the United States. Chromosomal abnormalities may be fatal to the fetus and are commonly found in a fetus when a pregnancy ends in miscarriage or stillbirth. Down syndrome Down Syndrome (Trisomy 21) Down syndrome is a chromosome disorder caused by an extra chromosome 21 that results in intellectual disability and physical abnormalities. Down syndrome is caused by an extra chromosome 21... read more Down Syndrome (Trisomy 21) (trisomy 21) is the most common chromosomal abnormality in babies born alive.

Several factors increase the risk of having a baby with a chromosomal abnormality:

Did You Know...

  • The chance of having a baby with Down syndrome before age 35 is about 1 in 590.

  • The chance of having a baby with Down syndrome at or after age 40 is about 1 in 100.

Some chromosomal abnormalities can only be detected with specialized testing. Most of these abnormalities are too small to be seen with a microscope, so they are sometimes called submicroscopic abnormalities (or copy-number variants). For example, a very small part of a chromosome may be missing (called a microdeletion), or a chromosome may have a very small extra part (called a microduplication). Approximately 6% of children with birth defects have these types of submicroscopic abnormalities. Tests called chromosomal microarray testing can detect microdeletions and microduplications. Doctors may offer microarray testing before birth in certain circumstances—for example, when birth defects are suspected in a fetus.

Table

Single-gene disorders

Risk also depends on whether the gene is located on the X chromosome. There are 23 pairs of chromosomes. One pair, the X and Y chromosomes (sex chromosomes), determines sex. All the rest of the chromosomes are called autosomal chromosomes. Women have two X chromosomes, and men have one X chromosome and one Y chromosome. If the abnormal gene is located on the X chromosome, the disorder it causes is called an X-linked X-Linked Inheritance Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or code for functional RNA molecules.... read more (sex-linked) disorder.

If boys inherit an abnormal gene for an X-linked disorder (on the X chromosome), they have the disorder even if the gene is recessive because they have only one X chromosome, which has an abnormal gene. They do not have a normal X chromosome to override the one with the abnormal gene. If a male fetus inherits a dominant X-linked gene, the pregnancy typically ends in miscarriage Miscarriage A miscarriage is a pregnancy loss before 20 weeks of pregnancy. Miscarriages are very common, especially early in pregnancy. Most of the time, the cause of a miscarriage is unknown, but it may... read more .

However, girls must inherit two abnormal genes to develop an X-linked disorder if the gene is recessive. If the abnormal gene is dominant, only one abnormal gene is required for the disorder to develop.

If the prospective mother and father are related, they are more likely to have the same mutation in one or more of the genes that cause autosomal recessive disorders. Thus, the risk of such disorders is increased.

Table

Neural tube defects

Neural tube defects Neural Tube Defects and Spina Bifida Neural tube defects are a certain type of birth defect of the brain, spine, and/or spinal cord. Neural tube defects can result in nerve damage, learning disabilities, paralysis, and death. The... read more are birth defects of the brain or spinal cord. (The brain and spinal cord develop from a part of the embryo called the neural tube). Examples are spina bifida (in which the spine does not completely close, sometimes exposing the spinal cord) and anencephaly (in which part of the brain and skull is missing). For most of these defects, the cause is a combination of abnormal genes and other factors. Other factors include

A few neural tube defects result from hereditary abnormalities in a single gene, from chromosomal abnormalities, or from exposure to certain medications or chemicals.

Abnormalities due to several factors

Some birth defects, such as cleft lip or palate Cleft Lip and Cleft Palate A cleft is an opening that can form in the lip (cleft lip), roof of the mouth (cleft palate), or both if the tissue does not join together completely during pregnancy. Cleft lip and cleft palate... read more Cleft Lip and Cleft Palate , result from abnormalities in one or more genes plus exposure to certain other factors, including substances in the environment. The abnormal gene makes the fetus more likely to develop a birth defect, but the birth defect usually does not develop unless the fetus is exposed to specific substances, such as certain illicit drugs or alcohol Overview of Substance-Related Disorders Medications and other substances, whether used for legitimate medical purposes, as a habit (for example, caffeine), or recreationally, are an integral part of everyday life for many people ... read more . Many common birth defects, such as heart malformations, are inherited in this way.

Family History Assessment

Any prospective parent can request genetic screening, but screening is particularly recommended when

  • One or both partners know they have a genetic abnormality.

  • Family members have a genetic abnormality.

  • Parents have racial or ethnic ancestry with an increased risk of certain disorders.

Because parent ethnicity or race is often complex and not well defined and because prenatal genetic testing is becoming much less expensive and quicker, some health care professionals are starting to screen all potential (and expectant) parents, regardless of ethnicity.

Some genetic disorders are not hereditary and thus cannot be identified by screening the parents.

To determine whether a couple has an increased risk of having a baby with a genetic disorder, doctors ask the couple about the following:

Information about three generations is usually needed. If the family history is complicated, information about more distant relatives may be needed. Sometimes doctors review the medical records of relatives who may have had a genetic disorder.

Genetic Carrier Screening

Carriers are people who have an abnormal gene for a disorder but who do not have any symptoms or visible evidence of the disorder.

In carriers, the abnormal gene is usually recessive—that is, two copies of the gene are needed to develop the disorder (see Recessive Disorders Recessive x-linked disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or code for functional RNA molecules.... read more ). Such carriers have one normal gene and one abnormal gene for the disorder.

Carrier screening usually consists of analyzing the DNA from a blood sample. But sometimes a sample of cells from the inside of the cheek is analyzed. People provide the sample by swishing a special fluid in their mouth, then spitting it into a specimen container, or by rubbing a cotton swab inside their cheek.

Ideally, carrier screening is done before a woman becomes pregnant. If it is done afterward and if it indicates that both partners have a recessive gene for the same disorder, they may decide to have prenatal diagnostic testing Prenatal Testing for Genetic Disorders and Birth Defects Prenatal testing for genetic disorders and birth defects involves testing a pregnant woman or fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including... read more . That is, the fetus may be tested for the disorder before birth.

Table

Abnormal Results of Genetic Carrier Testing

If genetic screening tests show a high risk of passing on a serious genetic abnormality, the couple can consider the following:

  • Using sperm from another man if the man has an abnormal gene

  • Using an egg from another woman if the woman has an abnormal gene

  • Having prenatal diagnostic testing to do genetic testing of an embryo or fetus during a pregnancy

  • Choosing not to become pregnant

If an abnormality is diagnosed, the doctor explains what is likely to happen to a fetus with that particular genetic abnormality or birth defect. Sometimes the parents are referred to a genetic specialist to discuss the issues. In some cases, the abnormality can be treated. In other cases, it is likely the fetus will die before birth or will have a limited life expectancy after birth. For some abnormalities, the child may have an intellectual disability, physical disability, or other medical problems.

Doctors will explain options, and prospective parents can then decide to continue the pregnancy or have a pregnancy termination Abortion Induced abortion is the intentional ending of a pregnancy by surgery or medications. A pregnancy may be ended by surgically removing the contents of the uterus or by taking certain medications... read more . Doctors may advise a woman to get prenatal care at a hospital that provides more extensive services for newborns and children with medical issues.

Parents should take time to absorb the information and should ask any questions they have.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Folacin , Folicet, Q-TABS
NOTE: This is the Consumer Version. DOCTORS: VIEW PROFESSIONAL VERSION
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